This invention relates to apparatus for feeding particulate material, such as coal particles to be burned in a coal combustion system in an electric power plant in which the coal is fed at a controlled rate from supply means to a discharge chute that delivers the coal to a furnace component. More particularly, the invention relates to gravimetric feeder apparatus whose rate of discharge is controllable in terms of the weight of material being discharged per unit of time, and especially to such gravimetric feeder apparatus adapted for installations where the supply means and discharge chute have flow paths with center lines closely spaced from another by a distance substantially less than the length of the horizontal span across which the coal must be transported to accomplish the continuous weighing by the gravimetric feed control system. Although the present invention is particularly adapted for feeding coal in power plants, the invention is useful in the feeding of particulate material in general. Although terms specific to coal will be used herein, it is not intended that the basic concept of the invention be restricted to feeding coal.
In many installations of the type to which the present invention is applicable, coal or other particulate material is stored in and supplied from one or more overhead storage means and fed downwardly by gravity to one or more locations where it is to be utilized. In a coal combustion system of a coal-burning power plant, the overhead storage means is usually a hopper or bunker, and the combustion apparatus to which the coal is supplied usually includes pulverizers, boiler stokers, or cyclone burners.
Known gravimetric feeders, such as that shown and described in U.S. Pat. No. 3,187,944, the disclosure of which is incorporated by reference herein and made a part hereof, are designed to feed coal by weight to such coal combustion systems.
Generally, such a gravimetric feeder receives the particulate coal in the form of what might be termed a continuous mass or columm moving in a fixed flow path, and delivers the coal at a controlled rate as free-falling or separately flowing particles. The rate of discharge is controllable in terms of the weight of the material being discharged. Since feeding by control of weight can be more accurate, such a feeder makes possible accurate control of the fuel-air ratio. A feeder of this type makes it possible to divide the total coal flow equally among several firing units of a single boiler.
Such a feeder utilizes a horizontal conveyor located within a housing adapted to contain gas at a pressure higher than ambient atmospheric air pressure. The conveyor includes an endless flexible belt and has an upper span or run that receives coal through an inlet in the vessel in the form of a solid column from the hopper. The upper belt run transports a layer of the coal at a level controlled by an adjustable leveling bar or valve across a weighing span of predetermined length in the upper belt run, which weighing span is supported by three spaced parallel rollers, the center roller being a weighing roller. The weighing roller thus carries one-half the weight of coal on the weighing span and the weight carried thereby is sensed by a lever scale, or the like, adjusted to a desired "balance" weight. If the weight sensed varies from that desired, then the weighing mechanism automatically moves a leveling bar to adjust the depth (and thus the weight) of coal transported on the belt.
Usually the gravimetric feeder includes as additional components a scraper to keep the return span of the belt as clean as possible and a cleanout conveyor to remove accumulated dust from the bottom of the housing. The cleanout conveyor ordinarily utilizes transverse metal flights that move longitudinally of a metal through. Normally, the cleanout conveyor needs to be operated only a few minutes each hour.
As a practical matter, in order to make possible the above operation, the length portion of the upper span of the belt from the center line of the flow path of the coal being fed from the hopper to the exit or drop-off zone at the opposite end of the belt must be substantial, at least about six feet for a gravimetric feeder of the type indicated above, to provide space for the mechanisms and other apparatus necessary to perform the weighing and control functions. Therefore, the center line of the coal column feeding to the feeder must be spaced horizontally for a substantial distance from the location at which the coal discharges from the feeder belt. Heretofore, the coal has been discharged directly downwardly by gravity into a discharge chute.
In some installations, however, such as existing installations, where it is necessary or desirable to install a gravimetric feeder the center line of the flow path of the column of coal to the feeder is coextensive with, or closely laterally spaced to, the center line of the flow path through the discharge chute, so that such spacing is therefore substantially less than the space indicated above as required between the feed and discharge portions of the belt. Incorporation of a gravimetric feeder into such an existing installation has not been possible.
The present invention, however, permits installation of a gravimetric feeder in such installations in which the flow paths of the hopper and discharge chute are closely spaced; and it also affords other features and advantages not obtainable in the prior art.